Area Forecast Discussion
Issued by NWS Salt Lake City, UT
Issued by NWS Salt Lake City, UT
885 FXUS65 KSLC 202124 AFDSLC Area Forecast Discussion National Weather Service Salt Lake City UT 324 PM MDT Fri Sep 20 2024 .SYNOPSIS...Low pressure crossing the Desert Southwest will bring showers and thunderstorms Saturday, primarily to southern Utah. A mostly dry front will cross northern portions of the area on Monday before high pressure moves in for much of the remainder of the upcoming week. && .SHORT TERM (Through 12Z/6AM Sunday)...A broad low is currently centered over southern California this afternoon and is making its way to the east. Ahead of the trough, a few showers have developed over southwest Utah, though they appear to be generally weak. The remainder of the area remains dry with just some scattered cumulus clouds and temperatures near normal for this time of year. By sunrise tomorrow, the center of the low is on track to move into southern Arizona. The position of the low will bring a bit of east to northeasterly flow to northern Utah during this time, which could result in some enhanced canyon winds over the prone areas of northern Utah, including parts of the Wasatch Front. Guidance overall has backed off on the strength of the winds owing to a relative lack of cold advection and a shorter period of easterly flow. As the low moves into Arizona, the threat of showers will spread across southern Utah, with enough instability for scattered coverage of precipitation overnight. With daytime heating, the threat of showers and storms will spread northward to include the higher terrain of central and northern Utah, primarily east of I-15, bringing isolated showers as far north as the southern half of the Wasatch Front. By Saturday evening, the low is expected to continue into New Mexico, allowing coverage of showers over the area to quickly diminish. .LONG TERM (After 12Z/6AM Sunday), Issued 429 AM MDT...To close out the weekend, deterministic global solutions and ensemble solutions begin the period in good agreement regarding the large- scale pattern. We kick things off with a closed low over/near the Colorado Front Range on Sunday, with lingering early Sunday cyclonic flow extending westward into extreme eastern Utah, with a very low (<15% chance) for a shower for these extreme eastern areas Sunday. Otherwise, Sunday is expected to be precipitation- free with temperatures just a shade below seasonal normals. The journey into the long term portion of the forecast is a short one before model spread is introduced, with deviating solutions arriving as early as Monday. First, we`ll dive into where solutions are aligned: there is agreement that a ridge will build offshore of the western CONUS with a trough diving down the periphery of the ridge across the Rocky Mountains Monday and Tuesday. Two forecast scenarios are in play regarding the evolution of this trough. There is a subset of the ensemble solution space (as well as the deterministic ECMWF) that closes a low off directly over Utah on Monday, with the low lingering around through Wednesday. This would result in cool, showery conditions for the start of the workweek. This outcome is represented by a minority of the solutions, and interestingly the NBM is more reflective of this solution, with mentionable precipitation across the southern half of Utah on Monday and closer to the Arizona border Tuesday. The alternate scenario is one with a progressive trough which would maintain seasonable temperatures and dry conditions for the early portion of the workweek. Ensemble spread then decreases (with an associated increase in forecast confidence) for the mid to late period, with consensus showing positive height anomalies gradually building across the Great Basin, resulting in a gradual warming and drying trend through the remainder of the week. While there are a few ensemble members that want to bring a trough through the northern tier and knock down the ridge, the overwhelming consensus maintains positive height anomalies/ridging across the region, resulting in stellar weather to kick off the official start of the Autumn season. && .AVIATION...VFR conditions are expected through the period. Northwesterly winds will likely transition to a period south to southeasterly drainage winds between 03-06Z, with a shift to west- northwest to northwest between 06-08Z. Northwest winds are likely to continue into the day Saturday. .REST OF UTAH AND SOUTHWEST WYOMING...Terrain based convection will continue across portions of central and southern Utah through the evening. Convection may linger across southwestern Utah into the overnight hours. More widespread showers and thunderstorms will develop across southern and eastern Utah Saturday afternoon and evening. && .FIRE WEATHER...An upper low currently centered over southern California will make its way east across the Desert Southwest through Saturday. This will bring enough moisture and instability to the area for development of showers and thunderstorms, primarily over southern Utah where coverage will be fairly widespread. The main threat with any storms that develop will be gusty downburst winds, but there is a chance of heavy rain that could lead to wetting rainfalls. Drier conditions will move in Sunday behind the exiting trough as temperatures stay near to 5F below normal for this time of year. A grazing storm system will then bring a mostly dry cold front to northern Utah Monday. High pressure will move in thereafter, bringing drier and warmer conditions. && .SLC WATCHES/WARNINGS/ADVISORIES... UT...None. WY...None. && $$ SHORT TERM/FIRE WEATHER...Traphagan LONG TERM...ADeSmet AVIATION...Kruse For more information from NOAA`s National Weather Service visit... http://weather.gov/saltlakecity